1. Field of the Invention
The present invention relates to a heating control system for a gas sensor of an internal combustion engine, in particular, to a method of preventing damage to the gas sensor.
2. Description of Related Art
An internal combustion engine has a gas sensor in an exhaust pipe for detecting concentration of gas such as oxygen included in exhaust gas discharged from an engine main body. Detection signals provided by the gas sensor are used in control of various parts of the engine main body.
A publicly used gas sensor is constituted with a base substrate made of an oxygen-ion conductive solid electrolyte such as zirconia. Such a gas sensor utilizes an attribute of the oxygen-ion to diffuse into the solid electrolyte. In the gas sensor, a cavity is formed so that the oxygen can pass between an outside of the substrate, where detected gas exists, and an inside of the substrate. For instance, the cavity is formed so that the cavity communicates with the outside of the substrate through a diffusion layer having diffusion resistance. The oxygen in the cavity is pumped by a pump cell, whose electrodes are formed on both sides of the solid electrolyte constituting a part of a peripheral wall of the cavity. Thus, limiting current flows between the electrodes. Concentration of the gas is measured based on the value of the limiting current.
In such a gas sensor, the solid electrolyte needs to be heated to activation temperature to enable the detection of the gas concentration. Therefore, the gas sensor is integrated with a heater for heating the solid electrolyte to the activation temperature. After the engine is started, a heating control system of the gas sensor performs activation energization control for energizing the heater with driving power capable of heating the gas sensor to the activation temperature. Normally, the maximum driving power is outputted in order to immediately monitor the exhaust gas passing through the exhaust pipe.
However, zirconia or the like, which is commonly used as a material of the solid electrolyte of the gas sensor, has a fragile nature. Since the gas sensor is disposed in the exhaust pipe through which the exhaust gas including a large amount of moisture passes, the gas sensor may be damaged due to rapid displacement of heat from the gas sensor to water if water droplets contact the gas sensor to form a state in which the water droplets can receive the heat from the gas sensor.
However, improvement of durability of the gas sensor by improving the structure of the gas sensor has a limitation.